The use of high-alloy temperature-resistant materials for the rotor blades in gas turbine jet engines, e.g. in a low-pressure turbine, causes high rim loads on the rotor rim and therefore brings about a comparatively severe impairment of the rotor disk life. The rotor blades have a relatively high specific weight and are, for example, precision castings of nickel- or cobalt-base alloys. These rotor blades are normally anchored by their multiply serrated root sections, so-called pine-cone or fir-tree roots, in correspondingly contoured axial slots in the rotor rim.
With the above-described type of blade and blade root, the ever present tolerances or variances in the production of the serrated root and slot profiles practically do not cause any problems with regard to blade centering when the blades are made of high alloy materials. This is true because the above mentioned high-alloy blade materials provide sufficient ductility and flexural elasticity to compensate for production variances through relative local root deformation. In operation, therefore, surface support or contact occurs as expected at predefined bearing surfaces between the respective root serration and its companion slot surface. Disadvantageously, in order to cope with production variances and associated blade centering problems, while also withstanding the loads induced by prevailing forces, the above-described construction results in a statically indeterminate (overdetermined) arrangement.
Another disadvantage of the above-described known construction is that the manufacture of the blade roots and axial slots is complex and involves laborious and expensive processes.
The use of rotor blades made of light engineering materials, e.g. temperature resistant ceramics, would reduce the rotor disk rim loads. However, because they have inadequate ductility and flexural elasticity and are often prone to brittle fracture, these materials practically prohibit the use of the above mentioned fir-tree or pine-cone roots and profiles. In other words, the light engineering materials would not allow root deformation of the type that is necessary to compensate for production variances as described above.
It is generally known per se to provide rotor blades with dove-tailed roots. This type of root, however, cannot achieve a rotor blade centering effect like that achieved by the serrated pine-cone or fir-tree root. For lack of proper centering, such rotor blades, especially if they have comparatively long airfoils, tend to assume intolerably skewed positions relative to the required blade incidence angle.